The consequences of refusing to submit to Athenian power and become part of the Delian League were horrific for the people of Melos.

Nonetheless, in exploring the debate between the representatives of the two sides which preceded these events, you have to keep the author as central. This is a debate scripted by the Athenian Thucydides during his exile from Athens. This context raises questions about whose point of view he was foregrounding and about his motivations as the author. The account of the debate tells us what Thucydides would like us to think about the Athenians and remember that they had exiled him, which might have influenced his views. The speeches he offers are imagined recreations of events he did not witness.

His version of the dialogue is that the Athenian and Melian representatives talked - the populace was not involved (my point here is that democracy was not evident in the debate at all). The general gist of the dialogue is that the Athenians repeatedly demanded Melian surrender while the Melians countered with a range of views - it would be better for you if we were non-aligned, Sparta will help us, we might win if you fight us, the gods will help us because "we are standing for what is right against what is wrong" (which provokes the reply you have quoted above).

Honestly, I would not overstate any perceived sense of fairness you have about the Athenians. Athens was acting in its interests, and if those interests helped others, that was nice, but I have never seen evidence that supports the view that fairness towards other states was a significant motivation.

It is my perception as historian that empires act in their own interests (or at least in the interests of the oligarchies ruling them). I regret to say that I have never seen evidence to contradict this pessimistic view.